1. Field of the Invention
The present invention relates to system and methods for coupling powered pusher vessels to unpowered vessels. More specifically, the present invention concerns a system for coupling a first vessel, such as a tugboat or other pusher vessel, with a second vessel, such as an unpowered barge, to minimize undesirable movement therebetween while allowing for lightering, i.e., compensating for changes in the relative vertical positions, or draft differential, of the vessels as weight is added or removed from one or both of the vessels.
2. Background of the Invention
Barges are used to transport cargo on water but have no self-propelling mechanisms and therefore rely primarily on tugboats or other pusher vessels for movement from one location to another. There exist many types of connections for coupling tugboats to barges, and the particular form of connection used depends on the marine environment in which the transportation takes place. A more secure coupling arrangement is required to maintain the connection of the tugboats to the barges in rough waters. For example, when a barge is used to transport oil, the coupling of the tugboat to the barge, the loading of the barge, and the actual pushing of the loaded barge take place on open sea and are subject to substantial waves. The tugboats must be securely connected to the barges to prevent the tugboats from being tossed against the barges, otherwise substantial damage and injury can occur.
One type of connection involves extendable ram devices on the tugboats which interconnect with receivers on the barges. This connection is achieved by the bow of the tugboat entering a generally U-shaped or V-shaped notch in the barge. Once the tugboat is within the notch, the port and starboard rams are extended from the tugboat into the corresponding receivers on the barge using a drive mechanism that generates sufficient force to securely hold the tug within the notch in the barge. This type of connection is suitable when the barge maintains a fixed load and the tugboat merely transports the barge. However, a tugboat must frequently transport a barge to a first destination where the barge receives a load, and then transport the barge to a second destination where the barge is unloaded. During the loading and unloading operations, the respective drafts of the tugboat and the barge will change relative to each other as a function of the changing load weight. For example, when the barge is empty, it will sit relatively high in the water. As the barge is loaded, the weight of the load will cause the barge to sit lower in the water. Because the weight of the tugboat does not change, its draft remains constant. If the tugboat were to remain fixedly connected to the barge during the loading operation, the weight of the load would push the tugboat down into the water, with possibly disastrous consequences. The tugboat must, however, maintain some degree of connection with the barge during loading or else wave or wind action may prevent subsequent realignment and reconnection. Therefore, the connection must permit the relative heights above the water of the tugboat and the barge to change as the weight of the load changes. The process by which the tugboat adjusts its position relative to the connected barge as the weight of the load is changed is called “lightering.”
Another type of connection provides receivers in the form of vertical channels on the port and starboard sides of the notch in the barge for receiving the rams. The vertical channels extend from the top to the bottom height of the barge and have a wedge-shaped configuration which opens into the notch (inboard). The rams are provided with coupler heads having a similar wedge-shaped configuration, and which are oriented on the rams to mate with the vertical channels. A series of teeth are arranged along the sidewalls of the vertical channels. Similarly, a series of teeth are arranged along the lateral sides of the coupler heads. As the rams are extended and brought into engagement with the vertical channels, their respective teeth interlock. The depth of the teeth permits sufficient engagement to substantially prevent relative vertical movement of the tugboat with respect to the barge.
To permit relative vertical movement of the barge with respect to the tugboat, the rams are retracted a sufficient distance so that the teeth of the coupler heads completely disengage from the teeth of vertical channels. With this type of connection, any contact between the coupler heads and the barge when disengaged can causes the rams to rotate, thereby misaligning the respective teeth and making reengagement difficult. Furthermore, complete disengagement results in a loss of longitudinal and tugboat roll control. Any time that the rams disengage from the receivers, there is a potential for mishap, especially in rough wave or wind action. More specifically, whenever the tugboat and barge are disconnected in this manner, the tugboat is allowed to roll in the notch.